It is well-known to those skilled in the art that electron guns are constructed in a variety of configurations, depending upon the application thereof. However, certain features are common to most electron guns, one of which is the existence of a plurality of axially disposed apertures disposed in electrode plates or grids to accommodate passage of the electron beam.
Structurally, a typical electron gun includes an annular cylinder or like configuration having a cathode at one end, a G1 electrode mounted forwardly of the cathode and a G2 electrode positioned forwardly of the G1 electrode. Both the G1 and G2 electrodes are provided with central apertures aligned with the electron emitting face of the cathode, the apertures providing openings to permit passage of the electron beam. The operating efficiency of an electron gun is affected by proper alignment of the G1 and G2 apertures. The more precise this alignment, the better the operating characteristics of the gun.
An example of a high-resolution color television electron gun is shown in U. S. Pat. No. 4,469,987 to Blacker et al, dated Sept. 4, 1984, and assigned to the assignee of this invention. The electron gun shown therein includes a G1 electrode, a G2 electrode, a G3 electrode and main focus electrodes. Each electrode is electrically isolated from the others. In addition, this patent shows an electron gun wherein a tetrode section has three coplanar beams created by three discrete cathodes. Therefore, each of the G1, G2, G3 and main focus electrodes include three apertures.
Regardless of whether the electron gun includes a single beam construction or a three-beam array, heretofore, the most prominent method of obtaining aperture alignment through the series of electrode plates has been to employ some form or another of a mandrel. Examples of such "mandrelling" of the electrode plates for aperture alignment are shown in U. S. Pat. Nos. 3,500,520 and 3,510,926 to Oess, dated March 17, 1970 and May 12, 1970, respectively.
Although aperture aligning mandrels can be machined to very accurate dimensions, such as on a lathe, theoretically precision alignment of the electrode plate apertures should be achieved. However, in actual practice, this is not always true. One problem resides in the simple fact of "mechanical spring back." In other words, the electrode plates conventionally are fabricated of metallic material and the mandrel actually is forced through at least some of the apertured plates relative to other plates. Upon removal of the mandrel, the mechanical nature of the metallic plates tend to move back to their original condition, even if slightly.
Another problem occurs in the heating and cooling cycles required during manufacture of the electron gun and/or the cathode ray tube. This heating and cooling of the metallic electrode plates also results in misalignment. For instance, the electrode plates may be anchored in a glass bead for permanent alignment. The mandrel is inserted through the apertures in the electrode plates and maintained in position during the heating step required to render the glass beads at least semi-molten. This may be on the order of 1,000 degrees Celsius. When the mandrel is removed, and the components are allowed to cool, misalignment occurs. It simply is not practical for any acceptable production rate to allow the mandrel to remain within the assembly during the cool-down period. Production efficiency and cost prohibits the use of a sufficient number of fixtures to allow such procedures to be performed.